CAES power plants have become effective contributors to a utilities generation mix as a source of peaking or intermediate energy and spinning reserve. CAES plants store off-peak energy from relatively inexpensive energy sources such as coal and nuclear base load plants by compressing air into storage devices such as underground caverns or reservoirs. Underground storage can be developed in hard rock, bedded salt, salt dome or acquifer media.
Following off-peak storage, the air is subsequently withdrawn from storage, heated, combined with fuel and combusted and expanded through expanders, i.e., turbines, to provide needed peak/intermediate power. Since inexpensive off-peak energy is used to compress the air, the need for premium fuels, such as natural gas and imported oil, is reduced by as much as about two-thirds compared with conventional gas turbines.
Compressors and turbines in CAES plants are each connected to a synchronous electrical machine such as a generator/motor device through respective clutches, permitting operation either solely of the compressors or solely of the turbines during the appropriate selected time periods. During off-peak periods (i.e., nights and weekends), the compressor train is driven through its clutch by the generator/motor. In this scheme the generator/motor functions as a motor, drawing power from a power grid. The compressed air is then cooled and delivered to underground storage.
During peak/intermediate periods, with the turbine clutch engaged, air is withdrawn from storage and then heated and expanded through a turbine to provide power to drive the generator/motor. In this scheme, the generator/motor functions as a generator, providing power to a power grid. To improve the CAES efficiency, waste heat from a low pressure turbine exhaust is used to preheat high pressure turbine inlet air in a recuperator. The compression process in a CAES plant is characterized by a much higher overall compression ratio than traditionally experienced in conventional gas turbines. This requires multistage compression with intercoolers in order to improve CAES plant efficiency.
The turbomachinery associated with a convention CAES plant has high pressure and low pressure turbines with high pressure and low pressure combustors, respectively. Fuel is mixed with compressed air and combusted at essentially constant pressure in these combustors, thus producing mixtures of products of combustion with high temperatures. The high temperature mixtures are then expanded in series through the high pressure and low pressure turbines, thereby performing work. Each turbine generally has an optimum expansion ratio (i.e., ratio of turbine input pressure to turbine output pressure) resulting in the highest possible efficiency for a specific turbine inlet temperature. The efficiency and optimum pressure ratio increase with increasing turbine inlet temperatures.
Turbine trains used in CAES systems have an overall expansion ratio which is the product of expansion ratios of the individual turbines which are serially connected. The overall expansion ratio of a turbine train comprising high and low pressure turbines is the ratio of turbine train input pressure (to a high pressure turbine) to turbine train output pressure (exhaust from a low pressure turbine), and generally ranges for CAES applications from 20 to a 100 or more.
Due to generally high air storage pressures, CAES plants are subject to high operating pressures unless the intake compressed air is throttled to a lower pressure. As pointed out in U.S. Pat. No. 4,885,912 issued Dec. 12, 1989, throttling the pressure from 60 bar and above, that may be encountered in high pressure turbines of CAES systems, is inefficient due to the energy of stored pressure in the compressed air that is lost. As the patent recognizes, one solution is to develop high pressure combustors which are yet unproven. A second solution proposed by the patent is to eliminate the high pressure combustor and transmit the heated compressed air from the recuperator directly to the high pressure turbine. While this latter proposal is efficient under operating conditions it does create some instabilities and under startup conditions it creates some inefficiencies.
Furthermore, to further improve the efficiency of the process it is desirable to run the low pressure turbine as hot as its materials will permit. Increasing the heat of the working gas in the low pressure turbine not only increases the efficiency of the low pressure turbine cycle but also increases the temperature of the exhaust gas and, through the heat recovered in the recuperator, the temperature of the compressed air entering the high pressure turbine, thus improving the efficiency of the high pressure turbine as well.
Accordingly, it is an object of this invention to improve the startup conditions of the CAES system that does not employ a high pressure combustor. Additionally, it is a further object of this invention to improve the efficiency of the CAES system by enabling the low pressure turbine to handle a higher temperature working gas.